TreasuryDirect password cannot autofill

TreasuryDirect is a US government service that lets users purchase US bonds from the Treasury directly (hence the name). It's... old school, shall we say. This is the login page, but unfortunately you cannot get to the password page without entering a valid account number. I've attached a screenshot of the password entry page.

The intention, presumably to stymie keyloggers, is that the user clicks the keys on the virtual keyboard to enter their password. Obviously, this is slow and rather error-prone, especially with a long, complex password generated by 1Password. (Side note: the password is not case sensitive, which is a little scary) Unfortunately, 1Password in the browser cannot autofill the password. I experimented a bit with tweaking the page's HTML, and the issue seems to be that the input has the attribute readonly="readonly". When I delete that attribute, 1Password can fill the field correctly.

Are there any existing solutions to allow 1Password to fill this password? Right now, I hack at the HTML manually each time I log in, though I might investigate getting a browser extension that can do this for me. Interestingly, there are a couple previous threads about this, but they're ancient.